Distortion

The Sigma 150mm f/2.8 is a dedicated macro lens and as such meant for accurate repro work. As a result it shows an absolutely negligible amount of distortion.

Vignetting

The Sigma is a full format lens and enjoys the usual sweet spot effect when used on an APS-C DSLR. Consequently the light falloff is significantly lower at ~0.75EV (f-stops) at max. aperture - this can be still be visible in very critical scenes. Stopping down to f/4 reduces the issue to a negligible degree already.

MTF (resolution)

The resolution characteristic of the Sigma lens is pretty impressive indeed. It is already very sharp across the image frame at max. aperture. Stopping down improves primarily the center performance which is lifted to excellent quality. Diffraction effects have a higher impact from f/11 onwards but it's still very sharp here and usable at f/16. f/22 is very soft on an APS-C DSLR and should be avoided.

Field curvature is basically absent. The centering quality of the tested sample was good albeit not perfect (which is typical for lenses with image stabilizer).

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic Aberrations (CAs)

The CAs (color shadows at harsh contrast transitions) are insignificantly low which adds to the excellent subjective sharpness perception.

Bokeh

The quality of the bokeh (out-of-focus blur) is a primary aspect for a tele lens and especially for a macro lens. The Sigma lens delivers good results here but it is no "cream machine". At max. aperture, the highlights have a perfectly circular shape. The "cat's eye" that we've seen in the corners within the full format scope are not an issue anymore on an APS-C DSLR.
There is a bit of an outlining effect but the inner highlight zone is very evenly rendered. The highlights remains pretty much circular till f/5.6.

The quality of the out-of-focus blur is quite nervous in the foreground (see the sample crop to the left below) but smoother although still not perfect in the (more critical) background.

Bokeh Fringing / Longitudinal Chromatic Aberrations (LoCA)

LoCAs (non-coinciding focal planes of the various colors), sometimes called "bokeh CAs" basically non-existent. This is an extremely rare characteristic - this is a super-apochromatic lens!

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs